The use of Fabry-Perot interferometers to measure the absolute length of a gap is known. Use of a ball lens to collimate light shining on a Fabry-Perot interferometer is needed for sensors measuring gaps exceeding about 30 um in order to maintain a uniform optical path length for all light rays and to assure a high percentage of the light reflected by the interferometer is captured by the fiber. However, if the light delivery fiber is not precisely centered on the ball lens axis or if the interferometer is not precisely perpendicular to the axis of the incident light transmitted by the ball lens, then the reflected light from the diaphragm does not re-enter the fiber because the reflected light spot that is re-imaged by the ball lens is not centered on the end of the input fiber. As a result, if insufficient light reflected from the sensor re-enters the fiber, the results from the Fabry-Perot interferometer-based sensor are compromised.
Accordingly, a Fabry-Perot interferometer-based sensor with a ball lens and alignment scheme that reflects high intensity light signals would provide benefits such as improved power budget, improved signal to noise ratio, and would be welcomed by the industry.